The beta-endorphin (Beta-E) immunoreactive peptides are highly concentrated in the anterior and intermediate lobes of pituitary and the hypothalamus. However, a great variation is observed in the relative amount of these three Beta-E like materials in different regions. In the hypothalamus and the intermediate pituitary, Beta-E predominates and Beta-LPH and pro-opiocortin are the minor components. In the anterior pituitary, the ratio of Beta-E/Beta-LPH is about 1. The result is in good agreement with the view that the pro-opiocortin may function as precursor for ACTH and Beta-E in the anterior pituitary and intermediate pituitary respectively. Abnormal behavior and endorphin disturbance have been reported in obese mice. In these obese mice, Beta-E, Beta-LPH and pro-opiocortin are all elevated in the pituitary but not in hypothalamus. This result raises the possibility that the abnormal Beta-E metabolism may be involved in development of abnormal behavior and endocrine disturbance in obese mice.